But the agency is staring down a real possibility that it might not be able to send people into space after next year.

That risk likely increased after Boeing discovered a problem in a new spacecraft system the company designed for NASA.

The issue — a fuel leak — appeared on June 2, as Ars Technica first reported, when Boeing test-fired four thrusters designed to propel the Starliner away from a potential launchpad emergency.

"Our team is off fixing those problems, and the result of that test series is that we will have a better and safer spacecraft," John Mulholland, Boeing's vice president and Starliner program manager, told reporters during a call on Wednesday.

Mulholland added that the time and effort spent finding the root cause of the leak and making a fix has delayed several future Starliner tests. Those delays include a test of the Starliner's emergency escape system. They also include two crewed test launches, the first of which is now pushed back five to six months to mid-2019.

NASA requires all of the tests to be completed before the agency will certify the Starliner for routine spaceflight.

"The average certification date was December 2019 for Boeing and January 2020 for SpaceX, according to the program's April 2018 analysis," the report states. But this assessment is based on an April 2018 analysis — two months before Boeing's surprise fuel leak.

How NASA astronauts could temporarily lose access to space

The possibility that astronauts might have no way to fly to space began when the US government decided to retire NASA's space shuttle fleet.

In July 2011, the final space shuttle mission landed back on Earth. It was the last American spacecraft certified to carry humans that reached the International Space Station (ISS).

Access to the ISS — a $150-billion, football-field-size laboratory in the sky — is crucial for researching technologies that could enable lunar and Mars missions, according to NASA. So to replace its space shuttle and avoid a worrisome flight gap, the agency created a multi-billion-dollar competition called the Commercial Crew Program.

The goal was to spur private American companies to design safer, more affordable spaceships for NASA. Boeing and SpaceX emerged as the two top winners.

A Soyuz spacecraft rocketing a three-person crew toward the space station on December 17, 2017.Shamil Zhumatov/ReutersWhile the companies have been working on their new ships, NASA has been paying Russia increasingly exorbitant sums to fly its astronauts to space. The US astronauts ride with cosmonauts on Soyuz spacecraft — currently the only ship that can taxi to and from the space station. (NASA is forbidden by law from working with China and its human spaceflight program.)

However, in anticipation of SpaceX and Boeing's new ships — which were originally supposed to be ready for flight in 2017 — NASA paused orders for seats on Soyuz. So now Russia is slated to fly its last US astronauts in 2019.

To prepare Soyuz missions beyond 2019, Russia wants three years' notice, NASA has said. So even if the agency made a Soyuz order today, it would take Russia until 2021 for that mission to be ready. (A NASA spokesperson told Business Insider in an email that "the three-year ask for Soyuz missions has been consistent with previous planning, and is not a new requirement.")

This has created a possibility that the US could temporary lose access to space. If neither Boeing or SpaceX can get a ship NASA-certified by the end of 2019, astronauts may be grounded until one of the companies meets NASA's requirements or Russia delivers a new Soyuz mission.

For now, Boeing and SpaceX both still appear to be on track for NASA certification in 2019, according to the GAO report. But the Commercial Crew Program is experimental, which means a problem could pop up at the last moment.

That's exactly what happened with Boeing's system.

"The issues that we have seen are not out of standing with typical development program challenges across all companies and all industries," Mulholland said. "These development programs are hard, especially for these human spacecraft vehicles."

About those leaky fuel valves

Boeing's leak problem first appeared during an on-the-ground "hot fire" test at NASA's White Sands Test Facility in New Mexico.

When a Starliner spaceship gets prepared for launch, it will be attached to a ring-shaped service module. Everything will then be put on top of an Atlas V rocket.

Inside that service module are multiple thrusters, four of which are designed to serve an emergency launchpad-escape system. If there's a problem with the rocket that Starliner is supposed to ride into orbit, these thrusters can quickly blast the ship and its crew away from the launchpad and toward safety.

A thruster designed to help Boeing's CST-100 Starliner blast away from a launchpad in an emergency.Aerojet RocketdyneEach thruster, built by Boeing subcontractor Aerojet Rocketdyne, has two valves — one for propellant and one for oxidizer — making eight valves in total. When mixed together with heat, the fuel rapidly burns and produces up to 40,000 lbs of thrust.

All four thrusters successfully fired for 1.5 seconds during the test, Boeing said, yet began leaking fuel right after the burn.

The company declined to answer Business Insider's emailed questions about whether the fuel-leak problem might endanger astronauts. However, it's possible this could lead to a fire outside the Starliner or perhaps even an explosion.

"Flight safety and risk mitigation are why we conduct such rigorous testing, and anomalies are a natural part of any test program," Boeing said in an email.

On Tuesday, Aviation Week reported that four of the eight valves stuck open to cause the leak. Mulholland confirmed that report on Wednesday.

"We've completed the root-cause investigation and determined what caused those valves to stick open," he said, adding that tweaks to both software and hardware are expected to fix the problem.

No back-up plan for reaching space?

Given the number of delays with the Commercial Crew Program and its experimental nature, it's anyone's guess if Boeing or SpaceX will close the spaceflight gap that's threatening NASA.

When asked if NASA is confident that it can avoid a loss of access to space, an agency spokesperson told Business Insider that the spacecraft and rockets "must meet the agency's safety and technical criteria before the companies will be certified to launch crews into space," and that "successfully meeting those requirements has always taken precedence over schedule."

Business Insider asked NASA what its back-up plan is for accessing the space station beyond 2019, should Boeing or SpaceX not deliver a certified spacecraft in time to avoid the gap. The space agency did not provide one.

"As part of its normal operations planning, NASA is continuing to assess multiple scenarios to ensure continued US access to the International Space Station," the spokesperson said. "The agency is working closely with its commercial partners and is preparing for potential schedule adjustments experienced during spacecraft development."

According to the GAO report published in July, NASA "currently does not have a contingency plan for how to ensure an uninterrupted presence on the ISS beyond 2019."

Mulholland suggested that new problems and resulting delays aren't unexpected, given the complexity of equipment necessary to safely launch people into space.

"We laid out a very challenging and aggressive schedule, and we have had several slips in that plan," he said on the call. "That said, our commitment hasn't wavered to make sure that we do everything that we laid out to do in our plan, which will ensure that — when we fly — we're going to fly with the utmost mission safety and success."

On Friday, NASA plans to announce the astronauts who will fly on upcoming crewed missions. The agency may also present a more current schedule for SpaceX and Boeing's certification flights.